1. Field of The Invention
The present invention relates to a water-proof photographic camera which can be used in rainy weather, at the sea-side, and in the water, and more particularly, to such a water-proof photographic camera with an automatic focusing device which includes a focus condition detecting circuit for detecting the focusing condition of the camera objective lens, or an automatic range finder circuit for detecting a camera-to-object distance and which automatically adjusts the focusing of the lens in accordance with the detected focusing condition or camera-to-object distance. The focus condition detecting circuit may be of any known type, for example, that which photoelectrically detects the contrast in the image of the object formed on a predetermined focal plane. Also the automatic range finder device may be of any known type, for example, that which projects a spot light onto the object and detects from what direction the light reflected by the object is incident on the camera. Such an automatic range finder is disclosed in detail, for example, in U.S. Pat. Nos. 3,723,003 and 4,065,778.
2. Description of the Prior Art
If it is intended to provide an automatic focusing device for a water-proof camera, the following problems will occur in underwater photography. It is generally known that the camera receives only a small amount of light from an object to be photographed, since a large amount of visible light rays from the sun are absorbed in the water. It is also a well known fact that the contrast in the image of an object to be photographed is low in the water due to the uniformity of the object brightness because of the scattering of light rays. Therefore, it is practically impossible to detect the camera-to-object distance or the focus condition by detecting the contrast in image of the object to be photographed. One approach to overcome this problem is to project a visible spot light on the object to be photographed for giving a better contrast to the object. However, in practice, it is difficult to obtain the proper contrast that is necessary for detecting the camera-to-object distance or the focus condition because a large amount of light projected to the object from the camera is reduced by materials suspended in the water between the camera and the object to be photographed and by the absorption of light rays in the water as described above. Also, in the case where the water-proof camera is provided with a range finding circuit that emits infrared light and detects the camera-to-object distance by detecting the light reflected from the object, the camera receives only a small amount of reflected light from the object and always detects that the object is at infinity, since a large amount of the infrared light is absorbed by the water. Further, when a visible or infrared light is projected to the object to be photographed for the range finding, a signal representing a false camera-to-object distance may sometimes be produced when the range-finder receives light reflected from suspended materials in the water or when the suspended material has an abnormally high brightness. Thus, in underwater photography, the range finding circuit and the focus condition detecting circuit can not function normally, resulting in out-of-focus photographic pictures caused by erroneous focusing of the taking-lens.
To cope with such problems, the camera may be arranged such that, besides the automatic focusing device, a manual focusing device is provided to enable focusing to a manually set camera-to-object distance and a selector switch is provided to alternatively select one of the automatic and manual focusing devices. Thus, it is intended that when a picture is taken in an air environment the selector switch is manually switched to select the automatic mode for automatic focusing, whereas the manual focusing mode may be selected for underwater photography. However, a photographer is apt to forget to operate such a switch upon changing the photography mode, thereby conducting underwater photography with the changeover switch set in the automatic mode resulting in a high probability of an out-of-focus picture. Also, conversely, photography in an air environment with the changeover switch set in the manual mode may result in a similarly undesirable photographic picture.